The oxygen reduction reaction (ORR) has been studied on Pt nanoparticles in alkaline solution using d.c. and a.c. techniques. When the Pt/C+PTFE active layer is deposited on a rotating disc electrode, the low-frequency behaviour is controlled by diffusion inside the solution. However, when it is deposited on a gas diffusion electrode, an inductive behaviour characteristic of a two-electrochemical-step mechanism is clearly exhibited. These results can be described with either an ECE mechanism (four-electron pathway) or an EEC mechanism (two-electron pathway followed by chemical decomposition of OH2- on the initial or another particle). In alkaline media, more OH2- is detected on bulk Pt than on the active layers. However, these species probably undergo chemical decomposition on Pt particles within the active layer, and the EEC mechanism may be preponderant on small nanoparticles. (C) 2003 Elsevier Ltd. All rights reserved.